An anti-windup system is an essential part of a process controller with integrating control action (mainly PI and PID controller) that significantly improves control quality (e.g. reference tracking quality). The process controller output either directly manipulates an actuator or provides a reference for a subsequent cascade controller. In both cases those systems have a certain input range that must to be respected. It is well known that a simple clamping of the controller output to this range without anti-windup system can lead to delayed controller reaction and poor control quality. This is caused by an unconstrained value of the integrator that can push the controller output far from the range. The functionality of the anti-windup system is to adjust the integrator value such that the controller output is within the range and such that it promptly reacts to disturbances and reference changes.
A standard anti-windup system has two main requirements. It must be able to adjust the controller integrator value and it must know an input range of a system connected to the controller output. Due to the first requirement and due to practical consideration, an anti-windup system is usually an integral part of a controller. However, there are cases, where it is not possible to simultaneously meet both requirements. This can happen when the communication between the controller and the system connected to its output is limited such that the controller does not know the correct range and the connected system cannot influence the controller integrator value by any means.
An example can be a widespread combination of a room temperature controller with a fan coil unit controller in buildings. The power of the fan coil unit changes with a fluctuating supply water temperature, but typically there is no way how to communicate this time variable limit to the room temperature controller. Either it does not accept such information or there is no communication channel for this. Another example can be controller retrofitting.